Efforts toward the synthesis of imidazolidinyl and benzimidazolidinyl N-heterocyclic carbenes in cyclophanes

Abstract

Cyclophanes are phenyl-containing macrocycles that were initially discovered in the early 1950s and are known mostly for their interesting geometrical properties. These macrocycles exhibit planar chirality within the structural framework, making them good candidates for ligands in asymmetric catalysis. Since their initial discovery, a wide variety of cyclophanes have been synthesized and used for many applications. These applications include use as chemical hosts for small molecules and ions, use as asymmetric ligands, and also use as organocatalysts. Our interest in cyclophanes is centered on the last two applications. More specifically, the goal of this research is to synthesize an N -heterocyclic [2.2] or [3.3] cyclophane carbene with hopes of exploring its reactivity as a catalyst and as a ligand. The study of N -heterocyclic carbenes was initiated by Wanzlick and separately by Öfele in the early 1960s after the initial hypothesis that the stability of carbenes could be enhanced by the presence of two flanking nitrogen atoms. Though the "free carbenes" were never isolated in these studies, Wanzlick verified their existence by trapping experiments. Later, in 1991, using the methods developed by Wanzlick, Arduengo and coworkers reported the first synthesis and isolation of a stable crystalline N -heterocyclic carbene, 1,3-diadamantyl-imidazol-2-ylidene. Following this initial report by Arduengo, several other reports that chronicle the reactivity and usage of N -heterocyclic carbenes, have been issued. Wellknown examples of the usage of these carbenes as precatalysts for transition metal-catalyzed reactions such as the Heck reaction, hydroformylation, hydrosilylation and olefin metathesis, are extensively documented in the literature. As organocatalysts, these carbenes are known to catalyze various C-C coupling reactions in basic media such as the Michael-Stetter reaction and the benzoin and formoin condensations. Transesterification reactions have also been shown to occur with great utility. Synthetic efforts toward the desired N -heterocyclic cyclophane carbenes is the subject of this dissertation.